It might also be developed into a general treatment for old age, when, just like mice, we lose up to a third of our muscle mass and power - one of the major reasons for the unsteadiness we experience later in life.

But the researchers from the University of Pennsylvania Medical Centre, USA, also warn that any such treatment could be used or abused by athletes or people wanting to enhance their looks.

Muscle strength

Young mice given the therapy in this study showed an increase in muscle strength of 15% over those rodents that did not get the boosting injection.

In older mice, the effect was even better with researchers seeing a 27% increase in strength over untreated muscle. This had the effect of fully restoring their strength to what it was in young adulthood, the Pennsylvania group said.

"Our results show that it may be possible to preserve muscle size and strength in old age using this approach," said Professor Lee Sweeney, who led the study.

"We're now looking to see whether the technique might also be used to increase muscle strength in diseases such as muscular dystrophy."

Gene therapy

A virus was used to carry two specific muscle-enhancing genes into the muscle cells of the mice. One gene coded for a growth factor chemical called IGF-I which is critical in the process of muscle repair. Another produced a promoter protein that drove high production levels of the growth factor.

In this experiment, the researchers selected an adeno-associated virus, or AAV, known to be highly efficient at introducing its genes into target cells.

The AAV was first stripped of its own disease-causing genes and then reloaded with the muscle boosting genes. The engineered virus was then injected directly into the muscles of the mice.

Satellite cells

Under normal circumstances, damaged muscles release quantities of IGF-I as an activation signal to neighbouring cells known as satellite cells.

These are muscle stem cells - master cells - that can become new tissue. Once they are activated, they migrate into the muscle and repair it.

The researchers theorised that age-related muscle loss might be the result of a decline in this signalling process and theorised that if they could amplify the IGF-I messages it might stimulate the repair mechanism. This appeared to be the case.

Abuse warning

Although excited by the findings, Professor Sweeney raised one concern: "The beneficial effects of this gene therapy could easily be used in humans for athletic or even cosmetic enhancements and not only for limiting age-related muscle loss or for treating diseases of the muscle."

The research was supported by grants from the US National Institutes of Health and US the Muscular Dystrophy Association.

The study was presented to the 38th annual meeting of the American Society for Cell Biology in San Francisco and will be published in the 22 December issue of the Proceedings of the National Academy of Sciences.